Embedded resistance unit



March 5, 1946. l. wcox E'rAL EMBEDDED RESISTANCE UNIT Filed Feb. 7, 1944 l u l l l l I l l l l l l l l u .l-

Patented Mar. 5, 1946 EMBEDDED RESISTANCE UNIT Irvin W. Cox, West Allis, and Lester D.

Drugmand,

Milwaukee, Wis., assignors to Cutler-Hammer, Inc., Milwaukee, Wls., a corporation o! Dela- Ware Application February 7, 1944, Serial No. 521,314 y 6 Claims.

This invention relates to improvements in embedded resistance units.

A primary object of the invention is to render such units moisture resistant and to insure a high ground resistance thereof.

Another object is to provide for attainment of the aforementioned desirable characteristics in such units by simple and relatively inexpensive means.

Another object of the invention is to provide an embedded resistance unit having a covering of Water repellent material capable of withstanding temperatures up tc 500 degrees F., whereby such unit is substantially unaffected by water or moisi ture or temperatures incident to use thereof.

Another object of the invention is to provide a metal backed embedded resistance unit wherein the cement suriace is covered by a relatively thin layer of a siliceous resin whichv is substantially waterproof, is capable of withstanding high temperatures, is chemically inert,. bonds well to such cement surface, increases surface smoothness, and may act as a vehicle for application of a pigment over the surface of the embedding cement.

Another and more specific object of the invention is tc provide a cement-embedded, metal backed resistance unit wherein the cement surface is covered by a relatively thin layer of methyl silicone or a compound including such material.

Other objects and advantages of the invention will hereinafter appear.

Heretofore it was found practically impossible to provide for resistors a vitreous enamel coating which was free from crazing or cracks; and in those cases wherein the resistor was embedded in an insulating cement it was found that the cement was inherently very porous. In both cases water or moisture had more or less free access to the resistor, and destruction of the resistor by electrolysis or chemical action was likely to occur.

In accordance with our invention the resistive conductor and the associated contacts or terminal members are secured in their proper positions by the embedding cement, and by applying a relatively thin coating or layer of methyl silicone over the embedding cement and properly maturing it, the embedding cement is made highly repellent to water or moisture. I

In the drawing, Figure 1 is a top plan view of one form of metal backed variable rheostat of the embedded resistor type wherein the normally exposed area of the embedding material is coated with a composition including methyl silicone to render the embedding material substantially proof against entrance of water or moisture.

Fig. 2 is a fragmentary sectional View, on the broken line 2--2 of Fig. l, illustrating in greater detail the construction of the rheostat, and

Fig. 3 is a top plan view of a xed resistance unit of the metal backed type, the resistor being embedded and the embedding material being coated in the manner shown in Fig. 2.

Referring first to Figs. 1 and 2, the numeral t designates a metal plate or pan which is drawn, spun or otherwise formed from a suitable blank to provide a relatively large concentric recess or depression (oi circular form, as illustrated). Plate 5 is then provided (on both sides and on the edges) with a vitreous enamel coating, designated by the numeral In practice we prefer to iirst provide plate 5 with a ground coating of vitreous enamel, and to then apply and re over the first coating a second coating having good electrical insulating characteristics.

The foregoing procedure has been set forth merely by way of example; it being understood that for certain purposes and under certain operating conditions (of the rheostat or resistance unit) it may be desirable to limit the enameling of plate 5 to a ground coating only, or to apply two ground coatings, or to employ combinations oi various-enamels and to apply many coatings, each coating being preferably only live-thousandths to six-thousandths of an inch in thickness. However, we have foundvit desirable in practice to keep the enamel coating 6 as thin as possible, provided that the electrical insulating characteristics alforded thereby meet the particular requirements. As is well known by those skilled in the art, a thin vitreous enamel coating oi the character specified retains a sulcient degree of elasticity or exibility to prevent the same from cracking or crazing under theheat shock incident to use of the article.

In the device of Figs. 1 and 2 we prefer to employ a multiplicity of like elements of approxin mately spool-shape. That is to say, each element l has an intermediate cylindrical portion ln and substantially square end portions lb and 1 (see Fig. 2). The smooth upper surfaces ld of the respective end portions 'lb are adapted to serve as stationary contacts or contact buttons to be engaged in a predetermined sequence by the opposite ends oi a bridging or circuit-completing contactor shown in dotted lines at 8 in Fig. l. We prefer to form the elements l of brass; and in practice groups of the same have attached to the at lower surfaces le (Fig. 2) of their respective end portions 'ic (as by silver soldering, welding or in any other suitable manner) the ends and intermediate loop portions of a plurality of approximately W-shaped resistive conductors; two sets of such resistive conductors being illustrated, and the same being respectively designated by the reference characters 9, I0, ll, I2, I3, and I4. The two sets 9 to I4, inclusive,'of resistive conductors are preferably of substantially the same shape or contour, but are composed of wires of different diameter or caliber, so that, for instance, the resistive conductors 9 will have the highest resistance values and the resistive conductors I 4 will have the lowest resistance values.

Moreover, the stationary contact surfaces or buttons 'ld associated with the two groups of resistive conductors 9 to it are oset from a true diametrically opposite relationship to each other, so that with the twelve resistive conductors employed an off position for contactor 3 is afforded as Well as twenty-live other positions which respectively provide for inclusion of different values of resistance in the circuit to be controlled. Thus, as shown in dotted lines in Fig. 1, contactor S is in olf position, whereas upon clockwise movement thereof the same will rst engage the button which we have marked a and the same will thereafter simultaneously engage the button marked b, whereby all of the resistance is included in the controlled circuit. Upon continued clockwise movement of contactor 8 the button c will be engaged to exclude from circuit the larger loop portion of the lowermost resistive conductor i4. Thereafter the larger loop portion of the uppermost resistive conductor ld will be excluded from circuit, after which the smaller loop portion of the lowermost resistive conductor I 4 will be excluded from circuit; and so on, until in the nal circuit-completing position of contacterli the same will bridge the contacts to which the circuit wires designated L1 and L2 are connected.

The contact members "l, certain of which have the resistors 9 to it and line wires L1 and L2 secured to the lower end portions thereof, as aforedescribed, are positioned in any suitable or well known manner within the recess in the enameled plate 5, and then the embedding material I is poured into said recess. The material I5 may consist of any suitable type or composition of insulating cement. However, we prefer to employ a cement composed of 62 per cent silica floats, 22 per cent water, and 16 per cent sodium silicate. After the cement is pc ured into said recess, to a level substantially below the upper end of the latter, a suitable quantity of quartz sand is distributed over the upper surface of the cement and the unit vibrated to effect removal of any entrapped air, and to effect a thorough coating of` the sand grains by the cement:

The embedding material is then baked; the baking cycle preferably consisting of a six hour bake at a temperature of 180 degrees F., a three hour bake at 300 degrees F., and a one hour bake at 600 degrees F., at the end of which period the embedding material will be matured. The contact buttons and the resistive conductors associated therewith will then be rigidly positioned or held in place by the insulating cement. After the unit has cooled the exposed surfaces 'Id of the contact buttons may, if necessary, be ground or otherwise jointly machined to insure a coplanar relationship thereof.

The resistance unit or rheostat is thereupon coated with the methyl silicone compound. We

'prefer to employ a compound having approximately the following percentages (by weight) of ingredients: 35 per cent of methyl silicone (having 71 per cent solids), 25 per cent of acetone. and 40 per cent of silica floats. These ingredients are mixed well and stirred constantly to maintain a substantially homogeneous mixture. The silica floats would tend to settle out of the mixture in the absence of constant stirring or agitation of the latter. This mixture which is quite liquid is then poured over the exposed surface of the embedding material of the resistance unit, the unit being preferably tilted and simultaneously rotated, or otherwise oriented, to insure complete coverage of the exposed surface of the cement, and to insure overlapping of the coating onto the inner enameled surface of the recess in plate 5; such coating being designated by numeral I6 in Fig. 2. The unit is then dried at about degrees C. for about six hours, andthe unit is then baked for about three hours at 250 degrees C. to effect complete curing of the aforementioned resinous coating.

Although we have herein described our preferred manner and means for moisture-proofing the embedding material or cement, it is to be understood that various modifications of the material and procedure may be employedwithout departing from the spirit and scope of our invention as defined by the appended claims. For instance, the percentages of the given ingredients of the resinous mixture may be varied throughout a relatively wide range. Pure or dilute methyl silicone may be employed without the other ingredients aforementioned, but with correspondingly lesss economy. Suitable materials other than silica oats may be employed as fillers; solvents other than acetone may be employed; the resinous coating composition may be painted, sprayed or otherwise applied to the embedding material; and the time and temperatures employed for curing the resinous coating may be substantially varied without sacrificing the advantages contemplated by us.

As heretofore stated, it is practically impossible to produce a vitreous enamel embedding material of the character described which is free of cracks and crazes; and in many instances where the vitreous enamel embedding material is initially free from such defects, it is found that after the resistance or rheostat units are placed in service cracks and crazes soon appear, due to the heat shock imparted to the embedding material by the hot resistive conductor. If a single drop of water is placed on one of such cracks and a resistance measurement taken a few minutes thereafter it is usually found that the ground resistance has dropped to quite a low value. In some instances the ground resistance has been found to have dropped below 5000 ohms unde` such conditions. Such a reading indicates that the crack in the cement embedding material has extended entirely through the thickness of the cement to the metal plate; and it further indicates that conduction between the resistive conductor and the metal of the plate is possible if the resistance unit is used in a highly humid or moist atmosphere.

When a coating of methyl silicone (or a composition including the same) is applied to such a unit the aforementioned cracks and crazes are lled with such coating material, and after maturing of the coating it is found that the ground resistance of the unit will remain very high, even though the coated surface of the embedding cement may be covered with water.

In a resistance unit wherein the resistive con ductor is embedded in an insulating cement it liii situ to mature the ing, tc tliei'eliy tender seid cement moisture resistant, whereby a high ground resistance of the unit is insured,

5. ln a resistance unit. in combination, a sheet metal plete having a recess formed therein, all

surfaces, ci seid plete lit-wing e reletively thin coating ci e. vitreous enamel ci gccc'l electrical insulating-,f character, e, resistive ccnciuctcr posi 'cloned within said recess, e vitreous, adhesive insulating cement also pcsiticnecl within saiol recess and adapted to embed said resistive con ductcr, Selci cement comprising aimerainliiiatellyI 62 per cent silica floats, 22 per cent Water enel it per cent sodium silicate enel the seine beine naked came, e, relatively thin coating ci e compound consisting of accresce mately 35 per cent by Weight oi methyl silicone intimately mixed with a solvent and et miei@ com pletely covering the initially expcsecl surface ci scid cement and-overlying the adjacent com tions of seid enameled plate, seid compound lieing also belted in situ tc completely cure the Same, enti said compound when cured being adapted te withstand l@frees F. without .cracking cr creclng tc thereby tender said cement moisture ieslstant, whereby en unusually high @round resistance olf the limit is insured under all ccncllticne.

temperatures up to 500 cleascent? 6. In a resistance unit, in combination, a sheet metal plate' having a recess formed therein, all surfaces of said plate having a relatively thin coating ci a vitreous enamel of 'good electrical insulating character, a resistive conductor positioned within said recess, a vitreous, adhesive insulating cement also positioned within said xecess and adapt to embed said resistive conduc ter, saicl cement ccmpilsing approximately 32 per cent silica, ificats, 22 pei' cent water and 18 per cent scclinin silicate emi the seme keine baked in situ to metete the relatively thin coating ci e. ccmpcunc, ccmci'lslnfg ccci'cnl mately 35, per cent ley weicht methyl eillccne having 7i pei' cent scllcls, 25 per cent ci? acetone, and et@ per cent ci silice nente, completely ccverine the initially expose@ .surface ci sale cement end overlying the Mlacent ccitions ci said encmelcci plate, seid cc-mpound Peeing also baked in situ to completely cure the came, enti calci compound when cured being adapted to render scid cement moisture iesistant, Wlieeby en "tinnsuelly high ground resistance ci tile nuit insured under all conditionneB W. een

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